Add a test for update_presence (#10033)

https://github.com/matrix-org/synapse/issues/9962 uncovered that we accidentally removed all but one of the presence updates that we store in the database when persisting multiple updates. This could cause users' presence state to be stale.

The bug was fixed in #10014, and this PR just adds a test that failed on the old code, and was used to initially verify the bug.

The test attempts to insert some presence into the database in a batch using `PresenceStore.update_presence`, and then simply pulls it out again.